There are two different patterns to the M57 grips depending on the age of the pistol. From 1946-1963 the grips had FNRJ for Federativna Narodna Republika Jugoslavija (Federal People's Republic of Yugoslavia). On pistols made in 1963-1992, the grips had the SFRJ instead for Socijalistička Federativna Republika Jugoslavija (Socialist Federal Republic of Yugoslavia). Essentially, they changed up the official country name in 1963 and so the grip styles were changed accordingly. My M57 was made in 1964 and has one of each grip panel, so some where along the way a grip was replaced with the older version.

As to why it's not in Cyrillic, the official language of Yugoslavia was Serbo-Croatian which was written in the Latin alphabet (the same alphabet we use in English). Bosnian, Croatian, Montenegrin and Slovenian are all written in the Latin alphabet. Serbian can be written in the Latin or the Cyrillic alphabet. Only Macedonian is only written in Cyrillic.

TL;DR: SFRJ is the abbreviation of the official name of Yugoslavia from 1963-1992. It's not written in Cyrillic because the official language at the time was written in the Latin alphabet.